Actuator plugs are used for downhole tool actuation. Generally, actuator plugs are conveyed downhole to land on the tool and actuate it. Actuator plugs can take various forms such as balls, darts, etc. Actuator plugs can be conveyed by gravity and/or fluid flow. In this application, the terms “plug” and “ball” are used interchangeably.
Recently, as described in U.S. Pat. Nos. 6,907,936 and 7,108,067 to Packers Plus Energy Services Inc., the assignee of the present application, wellbore treatment apparatus have been developed that include a wellbore treatment string including one or more openable port mechanisms that allow selected access to one or more zones in a well. The port mechanism employed includes a port through the string wall and a sleeve thereover with a sealable seat formed in the inner diameter of the sleeve. The sleeve may be moved to open or close the port by launching a plug, which can land in and seal against the seat and thereby create a pressure differential to drive the sleeve through the tubing string, such driving acts to open or close the port over which the sleeve is positioned. If more than one openable port mechanism is employed, a plurality of plugs can be used and/or one plug can actuate more than one sleeve. In one multi-sleeve system, the seat in each sleeve can be formed to accept a plug of a selected diameter but to allow plugs of lesser diameters to pass.
Once the pressure differential is dissipated, the plug may tend to lift off the seat and in fact may, by flow of fluids upwardly in the well, begin to move toward surface. If the wellbore treatment apparatus is used in a multi-leg well, the movement of plugs out of the apparatus and/or out of the wellbore leg in which they were employed may interfere with wellbore operations in other parts of the well.